As a method of industrial manufacture of propylene oxide (an epoxide), the chlorohydrine method or a direct oxidation method such as the halcon method or the peracetic acid method is used. However, the drawbacks of these methods are that they produce byproducts, and are two-stage manufacturing processes. As alternatives, various methods have been proposed of manufacturing propylene oxide by contact oxygen-oxidation (partial oxidation) of propylene (an alkene). However, these methods have performance problems, for example that the catalyst used has low selectivity for propylene oxide. Accordingly, these methods have not been implemented in industrial production.
Again, U.S. Pat. No. 5,623,090 (corresponding to Unexamined Japanese Patent Publication No. 8-127550/1996) discloses a method of using a vapor-phase oxidation reaction catalyst which contains gold and titanium oxide (titania) to oxygen-oxidize an alkene in the presence of hydrogen, thereby producing the corresponding epoxide. This method has high (about 90%) selectivity for the epoxide, but uses as starting material an alkene, which costs more than an alkane.
On the other hand, U.S. Pat. Nos. 4,990,632 and 5,008,412 disclose a continuous manufacturing method in which propane (an alkane) is dehydrogenated to produce propylene, and then the propylene is partially oxidized to produce propylene oxide. In this method, propane, which is cheaper than propylene, can be used as starting material. Further, this method recycles (reuses) unreacted propane and propylene. Again, U.S. Pat. No. 4,609,502 (corresponding to Unexamined Japanese Patent Publication No. 61-189256/1986) and U.S. Pat. No. 4,849,537 (corresponding to Unexamined Japanese Patent Publication No. 2-1449/1990), for example, disclose a method for producing an alkene by dehydrogenation of an alkane. In these methods, the alkene produced is then partially oxidized (ammoxidized) to produce a nitryl.
However, in the manufacturing methods disclosed in these two U.S. Patents, since the hydrogen which dehydrogenation produces along with the propylene is not consumed, recycling causes it to build up in the system of reaction. Accordingly, a feature of these methods is that they use a gas separation technique called PSA (pressure swing adsorption) to disperse or eliminate the hydrogen from the system of reaction. In other words, the complexity of the process of recycling unreacted hydrogen in these manufacturing methods makes it difficult to call them industrially advantageous.
For these reasons, a manufacturing method having high selectivity for an epoxide and a simplified process for recycling unreacted propane, etc., i.e., an industrially advantageous method for continuous production of an epoxide from an alkane, would be much welcomed.